(1) Field of the Invention
The present invention relates to a heavy duty pneumatic radial tire. More particularly, the invention relates to an improvement of durability of the heavy duty pneumatic tires for use under a high speed and a high internal pressure, for instance, aircraft radial tires and super high speed train radial tires.
(2) Related Art Statement
With the high performance of automobiles and the consolidation of the roads, the radial structure of the tires has been recently been progressively adopted and the percentage of the radial structure has been increasing. However, in the case of the aircraft tires and the super high speed train tires, the structural innovation of the tires are slow, and much have not examined therefor up to now, probably because of consideration upon the safety. Nevertheless, while the social demand for energy saving and the demand for the fuel mileage improvement have recently become greater, weight-reducing need in the tire parts has been also getting larger and larger. From this point of view, radial tires using aromatic polyamide fiber cords instead of steel cords as tire reinforcement have been examined. In particular, it is anticipated that a large scale of weight reduction can be accomplished by using aromatic polyamide fibers as high tenacity and high modulus cords in the carcass plies.
However, there is a big problem in the use of the aromatic polyamide fibers in the plural carcass ply layers. In other words, the fibers having high tenacity and high modulus possess the problems that the fibers have inactive molecular motion with respect to heat and are difficult to thermally shrink. For this reason, when a tire is produced by using such plural ply layers, the ply cords positioned in the inner layer or layers are likely to meander. This results in local unwinding of the twisting cords. Further, such a phenomenon is likely to occur particularly at a shoulder portion and a bead portion at which deformation of the tire is larger. This is because a bending operation is generally necessary in forming the plural carcass plies during the production of the tire, and difference in cord passing distance occurs in the inwardly positioned carcass cord layer or layers. On the other hand, in the case of the cords of fibers made of rayon, polyester, nylon, or the like as the material of the general use tire cords, no such meandering occurs because they thermally shrink during the vulcanization. Further, in the case of the conventionally used aromatic polyamide fibers, there is a problem that the cords are cut due to fatigue during use at portions where the cords are locally unwound. Therefore, tires merely employing the aromatic polyamide in the plural carcass layers can not be practically used.